The airports operator BAA has lost its appeal against a Competition Commission ruling that it must sell Stansted airport.

BAA, which operates Heathrow, has been resisting the sale since the original ruling in 2009 that it should sell three of the seven British airports it owned at the time. The latest verdict by the competition appeal tribunal could still be fought at the court of appeal, and BAA is considering its options.

The Competition Commission found in 2009 that BAA's dominance created problems which adversely affected passengers and airlines using BAA airports.

However, BAA claimed there had been material changes in its situation since then. The operator sold Gatwick, its second-largest airport, for £1.5bn in 2009. It is also set to sell Edinburgh airport to comply with the commission's mandate to shed one of its bigger Scottish airports, with a sale expected to go through this summer.

Stansted is the UK's fourth busiest airport, with 18.3 million passengers in 2011, and 550 aircraft movements per day. BAA has 1,420 staff at Stansted, out of more than 10,000 employees overall.

BAA has long argued that the two airports serve very different markets, with the predominantly short-haul, cheaper flights leaving Stansted.

A BAA spokesperson said: "We are disappointed by the decision of the competition appeal tribunal, which we will now carefully consider before making any further statements."

Laura Carstensen of the Competition Commission said: "We are very pleased that our decision has been upheld. While BAA is of course entitled to explore the available avenues for challenge, it is now surely time for BAA to accept our findings and proceed with the necessary divestments.

"BAA is in the process of selling Edinburgh airport following our ruling. We will now press ahead to ensure that this is followed promptly by the sale of Stansted airport, to the benefit of passengers and airlines."

Ryanair, which mainly operates out of Stansted, said it welcomed the decision, claiming that BAA had doubled passenger charges and presided over record traffic declines while delaying the sale. Spokesman Stephen McNamara said: "These repeated delays in the sale of Stansted must now end. Ferrovial [the 49.99% shareholder] and the other owners of Stansted are unfairly enriching themselves at the expense of UK passengers/visitors who are suffering higher charges and third-rate service."